build_image: honor USE flags from environment
build_image: honor USE flags from environment

BUG=n0ne
TEST=Ran the following command to test:

USE="-compat_wireless" ./build_image

Change-Id: I4b3fd12c1f62e1b2551045a5831a63d848fe2a3f

Review URL: http://codereview.chromium.org/5139003

andrejro/coreos--scripts,marineam/coreos-scripts,zhang0137/scripts,bcwaldon/coreos-scripts,zhang0137/scripts,trnubo/scripts,andrejro/coreos--scripts,smilart/scripts,mjg59/scripts,crawford/scripts,mischief/scripts,mischief/scripts,BugRoger/scripts,nicescale/scripts,crawford/scripts,mjg59/scripts,marineam/coreos-scripts,fivethreeo/scripts,andrejro/coreos--scripts,marineam/coreos-scripts,gtank/scripts,crawford/scripts,sigma/coreos-scripts,fivethreeo/scripts,mischief/scripts,glevand/coreos--scripts,BugRoger/scripts,endocode/scripts,bcwaldon/coreos-scripts,coreos/scripts,vmware/coreos-scripts,BugRoger/scripts,bcwaldon/coreos-scripts,trnubo/scripts,smilart/scripts,nicescale/scripts,trnubo/scripts,endocode/scripts,gtank/scripts,sigma/coreos-scripts,endocode/scripts,fivethreeo/scripts,glevand/coreos--scripts,gtank/scripts,mjg59/scripts,zhang0137/scripts,coreos/scripts,vmware/coreos-scripts,sigma/coreos-scripts,nicescale/scripts,vmware/coreos-scripts,smilart/scripts